UV-crosslinkable polyacrylates are known from the prior art and offer advantages over thermally crosslinkable systems. For example, UV-crosslinkable acrylate polymers can be applied extensively to a carrier and crosslinked in a controlled manner by controlling the UV radiation. Such dynamically controllable polymer systems represent a substantial advantage in a modern production structure if a complex product portfolio is to be produced starting from a small number of basic building blocks.
In conventional processes, Norrish type I and type II UV activators can be added for the UV crosslinking of polyacrylates. However, in the case of type I activators, competition reactions frequently occur when saturated polyacrylates are to be reacted. Norrish type II activators achieve crosslinking of unfunctionalized polyacrylates, but the degree of crosslinking is frequently low.
Other approaches of the prior art start from acrylate polymers containing vinyl double bonds for crosslinking. For the crosslinking of such polymers, the prior art proposes the use of electron beams. However, the use of electron beams is expensive and is sometimes accompanied by damage to the carriers onto which the polymer to be crosslinked is coated. Moreover, gel formation can occur on account of the vinyl double bonds even during the polymerization of the monomers to the acrylate polymers. Against this background, U.S. Pat. No. 5,391,406 and U.S. Pat. No. 5,416,127 describe a method in which vinyl groups are introduced subsequently by means of a polymer-analogous reaction. Such functionalized polymers can be coated from the melt and, after addition of a photoinitiator, can be crosslinked by means of UV light. However, in this process too, gelling during processing occurs in the case of high temperatures and under the influence of high shear forces. Moreover, polymer-analogous reactions are comparatively expensive.
Further problems arise when resins are added to the acrylate polymers in order to establish specific properties, such as particular tackiness, since such resins absorb UV light, so that crosslinking by means of UV light is limited by the layer thickness. Moreover, a large number of known photoinitiators lose their reactivity as a crosslinking starter when the initiator is used in a hot-melt process. Still other initiators sublime under thermal loading and in vacuo, for which reason such initiators likewise no longer ensure adequate crosslinking after a low-pressure extrusion step.
Some of the disadvantages mentioned above can be solved by using a polyfunctional α-cleaver. However, low molecular weight fragments also form when such an α-cleaver is used, which fragments can lead to undesirable contamination of the acrylate polymer. Moreover, the excited state of a large number of photoinitiators which permit a direct crosslinking reaction is extremely short-lived, so that the crosslinking step is not economical for energy reasons.
Accordingly, the object underlying the present invention is to provide an improved polyacrylate composition which is easy to obtain, can be processed in a hot-melt process, does not tend to gelling either during the hot-melt process or during coating, and can be crosslinked economically from the point of view of energy.